As display devices, the following display devices have been conventionally known: an impulse-type display device such as CRT (cathode-ray tube), for example, and a hold-type display device such as a liquid crystal display device, for example.
In the impulse-type display device, a lighting period during which an image is displayed and a blanking period during which no image is displayed are repeatedly alternated in each pixel. Thus, even in a case where a moving image is displayed, for example, a viewer rarely views an image blurring of a moving object in the moving image. This is because the blanking period is inserted at timing when an image corresponding to one screen is rewritten. Therefore, the viewer can distinguish the moving object from a background and view the moving image without experiencing discomfort.
On the other hand, in the hold-type display device, a brightness of each pixel is maintained during a one frame period (one vertical period) during which an image corresponding to a one screen is rewritten. In the hold-type display device, in a case where a moving image is displayed, a viewer views an image blurring of a moving object in the moving image. Specifically, the viewer views blurring of a contour of the moving object. Such a phenomenon is called a moving image blurring (pseudo contour), and it is thought that the image blurring is caused due to the following (i) and (ii): (i) incapability of a display condition of the pixel to instantaneously respond to a gray scale transition and (ii) viewer's visual tracking of the moving object.
Because the hold-type display device has a drawback that such a moving image blurring is caused in a moving image display, the impulse-type display device has been long employed in a display such as a television to perform a moving image display.
It has been recently strongly demanded that the display such as a television be thin in thickness and light in weight. In such a circumstance, the hold-type display device, which is easy to be thin in thickness and light in weight, has been increasingly employed to such a display.
A liquid crystal display device, in particular, has features of a reduced thickness, a light weight, and a low power consumption. The liquid crystal display device has been recently widely used, replacing CRT, in various fields such as a television, a monitor, a mobile device such as a mobile phone, and the like.
However, the liquid crystal display device is generally very inferior to other display devices such as CRT in terms of a response speed to a display signal. In the liquid crystal display device, a display gray scale is changed by causing a change in a voltage applied to a liquid crystal layer of the pixel that forms a display screen, and thereby causing a change in an alignment condition of liquid crystal molecules so as to cause a change in a transmittance of the pixel. In the liquid crystal display device, a response speed of the pixel is equal to an inverse of a time (response time) required for the alignment condition of the liquid crystal layer to reach an alignment condition corresponding to an applied voltage.
However, it takes a certain amount of time before the alignment condition of the liquid crystal layer to reach the alignment condition corresponding to the applied voltage. In a liquid crystal panel compatible with a frame frequency of 120 Hz per second, for example, it is intended that rewriting in each pixel is performed 120 times per second. However, there may be a case that it takes two or more frame to cause a response in the pixel.
This may cause a problem that a desired display gray scale cannot be realized in a recent liquid crystal display device with a large screen or a high definition display. This is because, in this liquid crystal display device, a driving time (writing time) of each pixel is so short that the writing time may not be long enough to fully cause a change in an alignment condition of a liquid crystal molecule in response to a change in an applied voltage.
A driving method called overshoot driving (overdriving), which is a driving method (tone transition emphasis process) of a liquid crystal display device, has been recently proposed as a technique of improving a response speed of a liquid crystal (see the patent literature 1, for example).
The tone transition emphasis process is a driving process that (i) applies an exaggerated voltage to a pixel in response to a gray scale transition, so as to speed up a response of a liquid crystal of the pixel, and thereby (ii) improves a response speed of the pixel.
Specifically, in a case where a gray scale is changed from a gray scale A to a gray scale B greater than the gray scale A, a voltage greater than a writing voltage for the gray scale B is applied to a pixel during a predetermined period. After this, the targeted writing voltage for the gray scale B is applied to the pixel. This speeds up a change in alignment of liquid crystal molecules, and thereby causes a increase in the response speed of the liquid crystal. As such, it is possible to further speed up the response speed of the pixel for the gray scale transition from the gray scale A to the gray scale B.
In contrast to the above case, in a case where the gray scale is changed from the gray scale A to a gray scale C less than the gray scale A, a voltage less than a writing voltage for the gray scale C is applied to the pixel during a predetermined period. This can bring about an effect similar to the effect obtained in the above case.